1. Field of Invention
The present invention relates to a method of forming a semiconductor device, and more particularly to a method of using a selective chemical vapor deposition (CVD) process to form a void-free metal layer.
2. Description of Related Art
As the dimension of a semiconductor device is getting smaller, the dimension of the gate structure and the thickness of the gate insulation layer are reduced accordingly. However, a leakage current occurs when the gate insulation layer of silicon oxide becomes thinner. To reduce the leakage current, a high dielectric constant (high-k) material is used to replace silicon oxide for forming the gate insulation layer. The gate of polysilicon may react with the high-k material to generate a Fermi-level pinning, so that the threshold voltage is increased and the performance of the device is affected. Therefore, a metal layer (i.e. so-called work function metal layer) is used as a gate, so as to avoid an increase in the threshold voltage and reduce the resistance of the device.
One known method of forming a metal gate is described below. First, a high-k material layer, a work function metal material layer and a polysilicon material layer are sequentially formed on a substrate. Thereafter, the above-mentioned layers are patterned to form a gate structure including, from bottom to top, a high-k layer, a work function metal layer and a polysilicon layer. Due to the limitation of the etching process during the patterning step, the gate structure is usually shaped as a trapezoid with a bottom wider than a top, and the sidewall thereof and the substrate form an inner included angle of 88 to 89 degrees. Afterwards, a spacer is formed on the sidewall of the gate structure. Source/drain regions are then formed in the substrate beside the gate structure. Further, an interlayer dielectric (ILD) layer is formed over the substrate, and the ILD layer is planarized to expose the surface of the gate structure. Thereafter, the polysilicon layer of the gate structure is removed, and a metal layer is filled in the exposed trench of the gate structure. However, since the gate structure is shaped as a trapezoid with a bottom wider than a top, when the metal layer is filled in the exposed trench of the gate structure by a sputtering process, poor metal gap fill issue is caused, voids are formed, and the reliability and performance of the device are accordingly affected.